Recovering lost 21 cm radial modes via cosmic tidal reconstruction

21 cm intensity mapping has emerged as a promising technique to map the large-scale structure of the Universe, at redshifts z from 1 to 10. Unfortunately, many of the key cross-correlations with the cosmic microwave background and photo-z galaxies have been thought to be impossible due to the foreground contamination for radial modes with small wave numbers. In this paper, we apply tidal reconstruction to the simulated 21 cm fields and recover the lost large-scale radial modes successfully. We estimate the detectability of the cross-correlation signals and find they can be detected at high significance with current 21 cm experiments. The tidal field reconstruction method opens up a new set of possibilities to probe the Universe and is extremely valuable not only for 21 cm surveys but also for cosmic microwave background and photometric-redshift observations.